Fuel cell electrocatalyst of Pt-Zn-Ni/Fe

ABSTRACT

A fuel cell catalyst containing platinum, zinc, and at least one of nickel and iron.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 application of PCT/US03/06613,filed on Mar. 6, 2003, which claims priority from U.S. ProvisionalPatent Application Ser. No. 60/362,198, filed Mar. 6, 2002. The entirecontents of these related applications are hereby incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to metal catalysts, especially tocatalysts which comprise platinum, zinc and at least one of nickel andiron, which are useful in fuel cell electrodes and other catalyticstructures.

2. Background Information

A fuel cell is an electrochemical device for directly converting thechemical energy generated from an oxidation-reduction reaction of a fuelsuch as hydrogen or hydrocarbon-based fuels and an oxidizer such asoxygen gas (in air) supplied thereto into a low-voltage direct current.Thus, fuel cells chemically combine the molecules of a fuel and anoxidizer without burning, dispensing with the inefficiencies andpollution of traditional combustion.

A fuel cell is generally comprised of a fuel electrode (anode), anoxidizer electrode (cathode), an electrolyte interposed between theelectrodes (alkaline or acidic), and means for separately supplying astream of fuel and a stream of oxidizer to the anode and the cathode,respectively. In operation, fuel supplied to the anode is oxidized,releasing electrons which are conducted via an external circuit to thecathode. At the cathode, the supplied electrons are consumed when theoxidizer is reduced. The current flowing through the external circuitcan be made to do useful work.

There are several types of fuel cells, including those havingelectrolytes of: phosphoric acid, molten carbonate, solid oxide,potassium hydroxide, and proton exchange membrane. A phosphoric acidfuel cell operates at about 160-220° C., and preferably at about190-200° C. This type of fuel cell is currently being used formulti-megawatt utility power generation and for co-generation systems(i.e., combined heat and power generation) in the 50 to several hundredkilowatts range.

In contrast, proton exchange membrane fuel cells use a solidproton-conducting polymer membrane as the electrolyte. Typically, thepolymer membrane is maintained in a hydrated form during operation inorder to prevent loss of ionic conduction which limits the operationtemperature typically to between about 70 and about 120° C. depending onthe operating pressure, and preferably below about 100° C. Protonexchange membrane fuel cells have a much higher power density thanliquid electrolyte fuel cells (e.g., phosphoric acid), and can varyoutput quickly to meet shifts in power demand. Thus, they are suited forapplications such as in automobiles and small scale residential powergeneration where quick startup is a consideration.

In some applications (e.g., automotive) pure hydrogen gas is the optimumfuel; however, in other applications where a lower operational cost isdesirable, a reformed hydrogen-containing gas is an appropriate fuel. Areformed-hydrogen containing gas is produced, for example, bysteam-reforming methanol and water at 200-300° C. to a hydrogen-richfuel gas containing carbon dioxide. Theoretically, the reformate gasconsists of 75 vol % hydrogen and 25 vol % carbon dioxide. In practice,however, this gas also contains nitrogen, oxygen, and, depending on thedegree of purity, varying amounts of carbon monoxide (up to 1 vol %).Although some electronic devices also reform liquid fuel to hydrogen, insome applications the conversion of a liquid fuel directly intoelectricity is desirable, as then a high storage density and systemsimplicity are combined. In particular, methanol is an especiallydesirable fuel because it has a high energy density, a low cost, and isproduced from renewable resources.

For the oxidation and reduction reactions in a fuel cell to proceed atuseful rates, especially at operating temperatures below about 300° C.,electrocatalyst materials are typically provided at the electrodes.Initially, fuel cells used electrocatalysts made of a single metal,usually platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir),osmium (Os), silver (Ag) or gold (Au) because they are able to withstandthe corrosive environment—platinum being the most efficient and stablesingle-metal electrocatalyst for fuel cells operating below about 300°C. While these elements were first used in fuel cells in metallic powderform, later techniques were developed to disperse these metals over thesurface of electrically conductive supports (e.g., carbon black) toincrease the surface area of the electrocatalyst which in turn increasedthe number of reactive sites leading to improved efficiency of the cell.Nevertheless, fuel cell performance typically declines over time becausethe presence of electrolyte, high temperatures and molecular oxygendissolve the electrocatalyst and/or sinter the dispersed electrocatalystby surface migration or dissolution/re-precipitation.

Although platinum is the most efficient and stable single-metalelectrocatalyst for fuel cells, it is costly and an increase inelectrocatalyst activity over platinum is necessary for wide scalecommercialization of fuel cell technology. The development of cathodefuel cell electrocatalyst materials faces longstanding challenges. Thegreatest challenge is the improvement of the electrode kinetics of theoxygen reduction reaction. In fact, sluggish electrochemical reactionkinetics have prevented attaining the thermodynamic reversible electrodepotential for oxygen reduction. This is reflected in exchange currentdensities of around 10⁻¹⁰ to 10⁻¹² A/cm² for oxygen reduction on, forexample, Pt at low and medium temperatures. A factor contributing tothis phenomenon include the fact that the desired reduction of oxygen towater is a four-electron transfer reaction and typically involvesbreaking a strong O—O bond early in the reaction. In addition, the opencircuit voltage is lowered from the thermodynamic potential for oxygenreduction due to the formation of peroxide and possible platinum oxideswhich inhibit the reaction. A second challenge is the stability of theoxygen electrode (cathode) during long-term operation. Specifically, afuel cell cathode operates in a regime in which even the most unreactivemetals are not completely stable. Thus, alloy compositions which containnon-noble metal elements may have a rate of corrosion which wouldnegatively impact the projected lifetime of a fuel cell. The corrosionmay be more severe when the cell is operating near open circuitconditions (which is the most desirable potential for thermodynamicefficiency).

Electrocatalyst materials at the anode also face challenges during fuelcell operation. Specifically, as the concentration of carbon monoxide(CO) rises above about 10 ppm in the fuel the surface of theelectrocatalyst can be rapidly poisoned. As a result, platinum (byitself) is a poor electrocatalyst if the fuel stream contains carbonmonoxide (e.g., reformed-hydrogen gas typically exceeds 100 ppm). Liquidhydrocarbon-based fuels (e.g., methanol) present an even greaterpoisoning problem. Specifically, the surface of the platinum becomesblocked with the adsorbed intermediate, carbon monoxide (CO). It hasbeen reported that H₂O plays a key role in the removal of such poisoningspecies in accordance with the following reactions:Pt+CH₃OH→Pt—CO+4H⁺+4e ⁻  (1);Pt+H₂O→Pt—OH+H⁺ +e ⁻  (2); andPt—CO+Pt—OH→2Pt+CO₂+H⁺ +e ⁻  (3).As indicated by the foregoing reactions, the methanol is adsorbed andpartially oxidized by platinum on the surface of the electrode (1).Adsorbed OH, from the hydrolysis of water, reacts with the adsorbed COto produce carbon dioxide and a proton (2,3). However, platinum does notform OH species well at the potentials fuel cell electrodes operate(e.g., 200 mV-1.5 V). As a result, step (3) is the slowest step in thesequence, limiting the rate of CO removal, thereby allowing poisoning ofthe electrocatalyst to occur. This applies in particular to a protonexchange membrane fuel cell which is especially sensitive to COpoisoning because of its low operating temperatures.

One technique for increasing electrocatalytic cathodic activity duringthe reduction of oxygen and electrocatalytic anodic activity during theoxidation of hydrogen is to employ an electrocatalyst which is moreactive, corrosion resistant, and/or more poison tolerant. For example,increased tolerance to CO has been reported by alloying platinum andruthenium at a 50:50 atomic ratio (see, D. Chu and S. Gillman, J.Electrochem. Soc. 1996, 143, 1685). The electrocatalysts proposed todate, however, leave room for further improvement.

BRIEF SUMMARY OF THE INVENTION

Briefly, therefore, the present invention is directed to a catalyst foruse in oxidation or reduction reactions, the catalyst comprisingplatinum, zinc, and at least one of nickel and iron.

The present invention is also directed to a supported electrocatalystpowder for use in electrochemical reactor devices, the supportedelectrocatalyst powder comprising a catalyst comprising platinum, zinc,and at least one of nickel and iron and electrically conductive supportparticles upon which the catalyst is dispersed.

The present invention is also directed to a fuel cell electrode, thefuel cell electrode comprising electrocatalyst particles and anelectrode substrate upon which the electrocatalyst particles aredeposited, the electrocatalyst particles comprising a catalystcomprising platinum, zinc, and at least one of nickel and iron.

The present invention is also directed to a fuel cell comprising ananode, a cathode, a proton exchange membrane between the anode and thecathode, and a catalyst comprising platinum, zinc, and at least one ofnickel and iron for the catalytic oxidation of a hydrogen-containingfuel or the catalytic reduction of oxygen.

The present invention is also directed to a method for theelectrochemical conversion of a hydrogen-containing fuel and oxygen toreaction products and electricity in a fuel cell comprising an anode, acathode, a proton exchange membrane therebetween, a catalyst comprisingplatinum, zinc, and at least one of nickel and iron, and an electricallyconductive external circuit connecting the anode and cathode, the methodcomprising contacting the hydrogen-containing fuel or the oxygen and thecatalyst to catalytically oxidize the hydrogen-containing fuel orcatalytically reduce the oxygen.

The present invention is also directed to an unsupported catalyst layeron a surface of a electrolyte membrane or an electrode, said unsupportedcatalyst layer comprising platinum, zinc, and at least one of nickel andiron.

The foregoing and other features and advantages of the present inventionwill become more apparent from the following description andaccompanying figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic structural view showing members of a fuel cell.

FIG. 2 is a cross-sectional side view of a fuel cell.

FIG. 3 is a photograph of an electrode array comprising thin-film alloycompositions deposited on individually addressable electrodes preparedas described in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a metal-containing substance havingelectrocatalytic activity for use in, for example, fuel cells (e.g., anelectrocatalyst). In one embodiment the metal-containing substance is analloy of the components. However, it is to be noted that the substance(e.g., electrocatalyst) may be a mixture of discrete amounts of thecomponents (e.g., a mixture of metal powders or a mixture of deposits),wherein a discrete amount of the components may comprise a singlecomponent or a combination of components (e.g., an alloy).

In general, it is desirable to decrease the concentration of noblemetals (especially platinum) to reduce the cost of an electrocatalyst.However, as the concentrations of noble metals are decreased, theelectrocatalyst may become more susceptible to corrosion, and/or theactivity may be diminished. Thus, it is desirable to achieve the mostactivity per weight percent of noble metals without compromising, forexample, the life cycle of the fuel cell in which the electrocatalyst isplaced (see, end current density/weight fraction of platinum as setforth in Tables A-C, infra). Additionally, the composition of thepresent invention is preferably optimized to limit noble metalconcentration while improving corrosion resistance and/or activity, ascompared to platinum.

The present invention is thus directed to a metal-containing substance(e.g., catalyst or alloy) that comprises platinum, zinc, and at leastone of nickel and iron. Furthermore, the catalyst of the presentinvention comprises amounts of platinum, zinc, and at least one ofnickel and iron which are sufficient for the metals, present therein, toplay a role in the catalytic activity and/or crystallographic structureof, for example, the alloy. Stated another way, the concentrations ofplatinum, zinc, and nickel and/or iron are such that the presence of themetals would not be considered an impurity. For example, when present,the concentrations of each of platinum, zinc, and nickel and/or iron areat least about 0.1, 0.5, 1, or even 5 atomic percent.

In one embodiment the catalyst of the present invention comprises atleast about 10 atomic percent of platinum. In the foregoing or otherembodiments the concentration of platinum may be no more than about 80atomic percent. Accordingly, the concentration of platinum may, in someembodiments, be between about 10 and about 80 atomic percent, andbetween about 15 and about 60 atomic percent.

In one embodiment the catalyst or the present invention comprises atleast about 2 atomic percent of zinc. In the foregoing or otherembodiments the concentration of zinc may be no more than about 70atomic percent. Accordingly, the concentration of zinc may, in someembodiments, be between about 2 and about 70 atomic percent, and betweenabout 5 and about 60 atomic percent.

In one embodiment the catalyst of the present invention comprises atleast about 5 atomic percent of nickel, iron or combination thereof. Inthe foregoing or other embodiments the concentration of nickel, iron, orcombination thereof may be no more than about 80 atomic percent.Accordingly, the concentration of nickel and/or iron may, in someembodiments, be between about 5 and about 80 atomic percent, and betweenabout 10 and about 70 atomic percent.

In view of the foregoing, in one embodiment the catalyst comprises aconcentration of platinum that is between about 10 and about 80 atomicpercent, a concentration of zinc that is between about 2 and about 70atomic percent, and a concentration of nickel, iron, or combinationthereof that is between about 5 and about 80 atomic percent. In anotherembodiment the catalyst comprises a concentration of platinum that isbetween about 15 and about 60 atomic percent, a concentration of zincthat is between about 5 and about 60 atomic percent, and a concentrationof nickel, iron, or combination thereof that is between about 10 andabout 70 atomic percent.

In this regard it is to be noted that in one embodiment the substance ofthe present invention consists essentially of the foregoing metals(e.g., impurities that do not play a role in the catalytic activityand/or crystallographic structure of the catalyst may be present to somedegree). However, in other embodiments it is possible that the substancemay comprise other constituents as intentional additions. For example,many metal alloys may comprise oxygen and/or carbon, either as animpurity or as a desired alloy constituent. In view of the foregoing,the alloys of the present invention, while maintaining the same relativeamounts of the constituents disclosed herein (i.e., platinum, zinc, andnickel and/or iron), may comprise less than 100 percent of the disclosedatoms. Thus, in several embodiments of the present invention the totalconcentration of the disclosed atoms is greater than about 70, 80, 90,95, or 99 atomic percent of the substance (e.g., electrocatalyst alloy).

Pt—Zn—Ni Alloy Electrocatalyst Compositions

In accordance with one embodiment of the present invention, the catalystcomprises platinum, zinc, and nickel. In another embodiment the catalystis an alloy that consists essentially of the platinum, zinc, and nickel.

In one Pt—Zn—Ni embodiment the concentration of each constituent metalmay vary through a range. For example, in one embodiment theconcentration of platinum is between about 10 and about 80 atomicpercent. In another embodiment the concentration of platinum is betweenabout 15 and about 50 atomic percent. In yet another embodiment theconcentration of platinum is between about 20 and about 35 atomicpercent. Similarly, the concentration of zinc in one embodiment isbetween about 5 and about 60 atomic percent. In another embodiment theconcentration of zinc is between about 15 and about 50 atomic percent.In yet another embodiment the concentration of zinc is between about 20and about 40 atomic percent. The concentration of nickel in oneembodiment is between about 10 and about 70 atomic percent. In anotherembodiment the concentration of nickel is between about 20 and about 60atomic percent. In yet another embodiment the concentration of nickel isbetween about 30 and about 55 atomic percent.

Additionally, it has been discovered that improved electrocatalyticactivity is achieved by controlling the relative concentrations of thevarious metals. For example, in one embodiment the concentration ofplatinum is between about 10 and about 80 atomic percent, theconcentration of zinc is between about 5 and about 60 atomic percent,and the concentration of nickel is between about 10 and about 70 atomicpercent. In another embodiment, the concentration of platinum is betweenabout 15 and about 50 atomic percent, the concentration of zinc isbetween about 15 and about 50 atomic percent, and the concentration ofnickel is between about 20 and about 60 atomic percent. In yet anotherembodiment the concentration of platinum is between about 20 and about35 atomic percent, the concentration of zinc is between about 20 andabout 40 atomic percent, and the concentration of nickel is betweenabout 30 and about 55 atomic percent. In a further another embodimentthe concentration of platinum is between about 20 and about 30 atomicpercent, the concentration of zinc is between about 5 and about 15atomic percent, and the concentration of nickel is between about 60 andabout 70 atomic percent.

Pt—Zn—Fe Alloy Electrocatalyst Compositions

In accordance with one embodiment of the present invention, the catalystcomprises platinum, zinc, and iron. In another embodiment the catalystis an alloy that consists essentially of the platinum, zinc, and iron.

In one Pt—Zn—Fe embodiment the concentration of each constituent metalmay vary through a range. For example, in one embodiment theconcentration of platinum is between about 10 and about 80 atomicpercent. In another embodiment the concentration of platinum is betweenabout 20 and about 60 atomic percent. In yet another embodiment theconcentration of platinum is between about 35 and about 50 atomicpercent. In one embodiment the concentration of zinc is between about 2and about 70 atomic percent. In another embodiment the concentration ofzinc is between about 5 and about 50 atomic percent. In yet anotherembodiment the concentration of zinc is between about 5 and about 35atomic percent. In one embodiment the concentration of iron is betweenabout 5 and about 80 atomic percent. In another embodiment theconcentration of iron is between about 10 and about 70 atomic percent.In yet another embodiment the concentration of iron is between about 20and about 60 atomic percent.

Additionally, it has been discovered that improved electrocatalyticactivity is achieved by controlling the relative concentrations of thevarious metals. For example, in one embodiment the concentration ofplatinum is between about 10 and about 80 atomic percent, theconcentration of zinc is between about 2 and about 70 atomic percent,and the concentration of iron is between about 5 and about 80 atomicpercent. In another embodiment the concentration of platinum is betweenabout 20 and about 60 atomic percent, the concentration of zinc isbetween about 5 and about 50 atomic percent, and the concentration ofiron is between about 10 and about 70 atomic percent. In yet anotherembodiment the concentration of platinum is between about 35 and about50 atomic percent, the concentration of zinc is between about 5 andabout 35 atomic percent, and the concentration of iron is between about20 and about 60 atomic percent. In a further embodiment the catalystcomprises between about 40 and about 60 atomic percent of platinum,between about 10 and about 30 atomic percent of zinc, and between about25 and about 50 atomic percent of iron. In an additional embodiment theconcentration of platinum is between about 20 and about 40 atomicpercent, the concentration of zinc is between about 20 and about 50atomic percent, and the concentration of iron is between about 25 andabout 40 atomic percent.

Formation of an Electrocatalyst Alloy

The electrocatalyst alloys of the present invention may be formed by avariety of methods. For example, the appropriate amounts of theconstituents may be mixed together and heated to a temperature above therespective melting points to form a molten solution of the metals whichis cooled and allowed to solidify. Typically, electrocatalysts are usedin a powder form to increase the surface area which increases the numberof reactive sites and leads to improved efficiency of the cell. Thus,the formed metal alloy may be transformed into a powder after beingsolidified (e.g., by grinding) or during solidification (e.g., sprayingmolten alloy and allowing the droplets to solidify). It may, however, beadvantageous to evaluate alloys for electrocatalytic activity in anon-powder form (see, Examples 1 and 2, infra).

To further increase surface area and efficiency, an electrocatalystalloy for use in a fuel cell may be deposited over the surface ofelectrically conductive supports (e.g., carbon black). One method forloading an electrocatalyst alloy onto supports typically comprisesdepositing metal precursor compounds onto the supports, and convertingthe precursor compounds to metallic form and alloying the metals using aheat-treatment in a reducing atmosphere (e.g., an atmosphere comprisingan inert gas such as argon). One method for depositing the precursorcompounds involves chemical precipitation of precursor compounds ontothe supports. The chemical precipitation method is typicallyaccomplished by mixing supports and sources of the precursor compounds(e.g., an aqueous solution comprising one or more inorganic metallicsalts) at a concentration sufficient to obtain the desired loading ofthe electrocatalyst on the supports and then precipitation of theprecursor compounds is initiated (e.g., by adding an ammonium hydroxidesolution). The slurry is then typically filtered from the liquid undervacuum, washed with deionized water, and dried to yield a powder thatcomprises the precursor compounds on the supports.

Another method for depositing the precursor compounds comprises forminga suspension comprising a solution and supports suspended therein,wherein the solution comprises a solvent portion and a solute portionthat comprises the constituents of the precursor compound(s) beingdeposited. The suspension is frozen to deposit (e.g., precipitate) theprecursor compound(s) on the particles. The frozen suspension isfreeze-dried to remove the solvent portion and leave a freeze-driedpowder comprising the supports and the deposits of the precursorcompound(s) on the supports.

The temperature reached during the thermal treatment is typically atleast as high as the decomposition temperature(s) for the precursorcompound(s) and not be so high as to result in degradation of thesupports and agglomeration of the supports and/or the electrocatalystdeposits. Typically the temperature is between about 60° C. and about1100° C. Inorganic metal-containing compounds typically decompose attemperatures between about 600 and about 1000° C.

The duration of the heat treatment is typically at least sufficient tosubstantially convert the precursor compounds to the desired state. Ingeneral, the temperature and time are inversely related (i.e.,conversion is accomplished in a shorter period of time at highertemperatures and vice versa). At the temperatures typical for convertingthe inorganic metal-containing compounds to a metal alloy set forthabove, the duration of the heat treatment is typically at least about 30minutes. In one embodiment, the duration is between about 2 and about 8hours.

Unsupported Catalyst or Alloys in Electrode/Fuel Cell Applications

It is to be noted that, in another embodiment of the present invention,the metal substance (e.g., catalyst or alloy) may be unsupported; thatis, it may be employed in the absence of a support particle. Morespecifically, it is to be noted that in another embodiment of thepresent invention a metal catalyst or alloy, comprising platinum, zinc,and nickel and/or iron, may be directly deposited (e.g., sputtered)onto, for example, (i) a surface of one or both of the electrodes (e.g.,the anode, the cathode or both), and/or (ii) one or both surfaces of apolyelectrolyte membrane, and/or (iii) some other surface, such as abacking for the membrane (e.g., carbon paper).

In this regard it is to be further noted that each component (e.g.,metal) of the catalyst or alloy may be deposited separately, each forexample as a separate layer on the surface of the electrode, membrane,etc. Alternatively, two or more components may be deposited at the sametime. Additionally, in the case of an alloy, the alloy may be formed andthen deposited, or the components thereof may be deposited and then thealloy subsequently formed thereon.

Deposition of the component(s) may be achieved using means known in theart, including for example known sputtering technique (see, e.g., PCTApplication No. WO 99/16137). Generally speaking, however, in oneapproach sputter-deposition is achieved by creating, within a vacuumchamber in an inert atmosphere, a voltage differential between a targetcomponent material and the surface onto which the target component is tobe deposited, in order to dislodge particles from the target componentmaterial which are then attached to the surface of, for example, anelectrode or electrolyte membrane, thus forming a coating of the targetcomponent thereon. In one embodiment, the components are deposited on apolymeric electrolyte membrane, including for example (i) a copolymermembrane of tetrafluorethylene and perfluoropolyether sulfonic acid(such as the membrane material sold under the trademark NAFION), (ii) aperfluorinated sulfonic acid polymer (such as the membrane material soldunder the trademark ACIPLEX), (iii) polyethylene sulfonic acid polymers,(iv) polyketone sulfonic acids, (v) polybenzimidazole doped withphosphoric acid, (vi) sulfonated polyether sulfones, and (vii) otherpolyhydrocarbon-based sulfonic acid polymers.

It is to be noted that the specific amount of each metal or component ofthe catalyst or alloy may be controlled independently, in order totailor the composition to a given application. In some embodiments,however, the amount of each deposited component may be less than about 5mg/cm² of surface area (e.g., electrode surface area, membrane surfacearea, etc.), less than about 1 mg/cm², less than about 0.5 mg/cm², lessthan about 0.1 mg/cm², or even less than about 0.05 mg/cm².Alternatively, in some embodiments the amount of the depositedcomponent, or alloy, may range from about 0.5 mg/cm² to less than about5 mg/cm², or from about 0.1 mg/cm² to less than about 1 mg/cm².

It is to be further noted that the specific amount of each component,and/or the conditions under which the component is deposited, may becontrolled in order to control the resulting thickness of the component,or alloy, layer on the surface of the electrode, electrolyte membrane,etc. For example, as determined by means known in the art (e.g.,scanning electron microscopy or Rutherford back scatteringspectrophotometric method), the deposited layer may have a thicknessranging from several angstroms (e.g., about 2, 4, 6, 8, 10 or more) toseveral tens of angstroms (e.g., about 20, 40, 60, 80, 100 or more), upto several hundred angstroms (e.g., about 200, 300, 400, 500 or more).Additionally, after all of the components have been deposited, and/oralloyed (or, alternatively, after the alloy has been deposited), thelayer of the multi-component metal substance of the present inventionmay have a thickness ranging from several tens of angstroms (e.g., about20, 40, 60, 80, 100 or more), up to several hundred angstroms (e.g.,about 200, 400, 600, 800, 1000, 1500 or more). Thus, in differentembodiments the thickness may be, for example, between about 10 andabout 500 angstroms, between about 20 and about 200 angstroms, andbetween about 40 and about 100 angstroms.

It is to be still further noted that in embodiments wherein a catalystor alloy (or the components thereof) are deposited as a thin film on thesurface of, for example, an electrode or electrolyte membrane, thecomposition of the deposited catalyst or alloy may be as previouslydescribed herein.

Incorporation of the Electrocatalyst Compositions in a Fuel Cell

Although the alloy compositions of the present invention can be used inany type of fuel cell (e.g., phosphoric acid, molten carbonate, solidoxide, potassium hydroxide, and proton exchange membrane), they areparticularly suited for use in proton exchange membrane fuel cells. Asshown in FIGS. 1 and 2, a fuel cell, generally indicated 21, comprises afuel electrode (anode) 2 and an air electrode, oxidizer electrode(cathode) 3. In between the electrodes 2 and 3, a proton exchangemembrane 1 serves as an electrolyte and it is usually a strongly acidicion exchange membrane such as a perfluorosulphonic acid-based membrane.Preferably, the proton exchange membrane 1, the anode 2, and the cathode3 are integrated into one body to minimize contact resistance betweenthe electrodes and the proton exchange membrane. Current collectors 4and 5 engage the anode and the cathode, respectively. A fuel chamber 8and an air chamber 9 contain the respective reactants and are sealed bysealants 6 and 7, respectively.

In general, electricity is generated by hydrogen-containing fuelcombustion (i.e., the hydrogen-containing fuel and oxygen react to formwater, carbon dioxide and electricity). This is accomplished in theabove-described fuel cell by introducing the hydrogen-containing fuel Finto the fuel chamber 8, while oxygen O (preferably air) is introducedinto the air chamber 9, whereby an electric current can be immediatelytransferred between the current collectors 4 and 5 through an outercircuit (not shown). Ideally, the hydrogen-containing fuel is oxidizedat the anode 2 to produce hydrogen ions, electrons, and possibly carbondioxide gas. The hydrogen ions migrate through the strongly acidicproton exchange membrane 1 and react with oxygen and electronstransferred through the outer circuit to the cathode 3 to form water. Ifthe hydrogen-containing fuel F is methanol, it is preferably introducedas a dilute acidic solution to enhance the chemical reaction, therebyincreasing power output (e.g., a 0.1 M methanol/0.5 M sulfuric acidsolution).

To prevent the loss of ionic conduction in the proton exchangemembranes, they typically remain hydrated during operation of the fuelcell. As a result, the material of the proton exchange membrane istypically selected to be resistant to dehydration at temperatures up tobetween about 100 and about 120° C. Proton exchange membranes usuallyhave reduction and oxidation stability, resistance to acid andhydrolysis, sufficiently low electrical resistivity (e.g., <10 Ω·cm),and low hydrogen or oxygen permeation. Additionally, proton exchangemembranes are usually hydrophilic. This ensures proton conduction (byreversed diffusion of water to the anode), and prevents the membranefrom drying out thereby reducing the electrical conductivity. For thesake of convenience, the layer thickness of the membranes is typicallybetween 50 and 200 μm. In general, the foregoing properties are achievedwith materials which have no aliphatic hydrogen-carbon bonds, which, forexample, are achieved by replacing hydrogen with fluorine or by thepresence of aromatic structures; the proton conduction results from theincorporation of sulfonic acid groups (high acid strength). Suitableproton-conducting membranes also include perfluorinated sulfonatedpolymers such as NAFION and its derivatives produced by E.I. du Pont deNemours & Co., Wilmington, Del. NAFION is based on a copolymer made fromtetrafluoroethylene and perfluorovinylether, and is provided withsulfonic groups working as ion-exchanging groups. Other suitable protonexchange membranes are produced with monomers such as perfluorinatedcompounds (e.g., octafluorocyclobutane and perfluorobenzene), or evenmonomers with C—H bonds which, in a plasma polymer, do not form anyaliphatic H atoms which could constitute attack sites for oxidativebreakdown.

The electrodes of the present invention comprise the electrocatalystcompositions of the present invention and an electrode substrate uponwhich the electrocatalyst is deposited. In one embodiment theelectrocatalyst alloy is directly deposited on the electrode substrate.In another embodiment the electrocatalyst alloy is supported onelectrically conductive supports and the supported electrocatalyst isdeposited on the electrode substrate. The electrode may also comprise aproton conductive material that is in contact with the electrocatalyst.The proton conductive material may facilitate contact between theelectrolyte and the electrocatalyst, and may thus enhance fuel cellperformance. Preferably, the electrode is designed to increase cellefficiency by enhancing contact between the reactant (i.e., fuel oroxygen), the electrolyte and the electrocatalyst. In particular, porousor gas diffusion electrodes are typically used since they allow thefuel/oxidizer to enter the electrode from the face of the electrodeexposed to the reactant gas stream (back face), and the electrolyte topenetrate through the face of the electrode exposed to the electrolyte(front face), and reaction products, particularly water, to diffuse outof the electrode.

The electrically conductive support particles typically comprise aninorganic material such as carbon. However, the support particles maycomprise an organic material such as an electrically conductive polymer(see, U.S. Pat. Appln. 2002/0132040 A1). Carbon supports may bepredominantly amorphous or graphitic and they may be preparedcommercially, or specifically treated to increase their graphitic nature(e.g., heat treated at a high temperature in vacuum or in an inert gasatmosphere) thereby increasing corrosion resistance. For example, it maybe oil furnace black, acetylene black, graphite paper, carbon fabric, orcarbon aerogel. A carbon aerogel preferably has an electricalconductivity of between 10⁻² and 10³¹ Ω⁻¹·cm⁻¹ and a density of between0.06 and 0.7 g/cm³; the pore size is between 20 and 100 nm (porosity upto about 95%). Carbon black support particles may have a Brunauer,Emmett and Teller (BET) surface area up to about 2000 m²/g. It has beenreported that satisfactory results are achieved using carbon blacksupport particles having a high mesoporous area, e.g., greater thanabout 75 m²/g (see, Catalysis for Low Temperature Fuel Cells Part 1: TheCathode Challenges, T. R. Ralph and M. P. Hogarth, Platinum Metals Rev.,2002, 46, (1), p. 3-14). Experimental results to date indicate that asurface area of about 500 m²/g is preferred.

Preferably, the proton exchange membrane, electrodes, andelectrocatalyst materials are in contact with each other. This istypically accomplished by depositing the electrocatalyst either on theelectrode, or on the proton exchange membrane, and then placing theelectrode and membrane in contact. The alloy electrocatalysts of thisinvention can be deposited on either the electrode or the membrane by avariety of methods, including plasma deposition, powder application (thepowder may also be in the form of a slurry, a paste, or an ink),chemical plating, and sputtering. Plasma deposition generally entailsdepositing a thin layer (e.g., between 3 and 50 μm, preferably between 5and 20 μm) of an electrocatalyst composition on the membrane usinglow-pressure plasma. By way of example, an organic platinum compoundsuch as trimethylcyclopentadienylplatinum is gaseous between 10⁻⁴ and 10mbar and can be excited using radio-frequency, microwaves, or anelectron cyclotron resonance transmitter to deposit platinum on themembrane. According to another procedure, electrocatalyst powder isdistributed onto the proton exchange membrane surface and integrated atan elevated temperature under pressure. If, however, the amount ofelectrocatalyst particles exceeds about 2 mg/cm² the inclusion of abinder such as polytetrafluoroethylene is common. Further, theelectrocatalyst may be plated onto dispersed small support particles(e.g., the size is typically between 20 and 200 Å, and more preferablybetween about 20 and 100 Å). This increases the electrocatalyst surfacearea which in turn increases the number of reaction sites leading toimproved cell efficiency. In one such chemical plating process, forexample, a powdery carrier material such as conductive carbon black iscontacted with an aqueous solution or aqueous suspension (slurry) ofcompounds of metallic components constituting the alloy to permitadsorption or impregnation of the metallic compounds or their ions on orin the carrier. Then, while the slurry is stirred at high speed, adilute solution of suitable fixing agent such as ammonia, hydrazine,formic acid, or formalin is slowly added dropwise to disperse anddeposit the metallic components on the carrier as insoluble compounds orpartly reduced fine metal particles.

The loading, or surface concentration, of an electrocatalyst on themembrane or electrode is based in part on the desired power output andcost for a particular fuel cell. In general, power output increases withincreasing concentration; however, there is a level beyond whichperformance is not improved. Likewise, the cost of a fuel cell increaseswith increasing concentration. Thus, the surface concentration ofelectrocatalyst is selected to meet the application requirements. Forexample, a fuel cell designed to meet the requirements of a demandingapplication such as an extraterrestrial vehicle will usually have asurface concentration of electrocatalyst sufficient to maximize the fuelcell power output. For less demanding applications, economicconsiderations dictate that the desired power output be attained with aslittle electrocatalyst as possible. Typically, the loading ofelectrocatalyst is between about 0.01 and about 6 mg/cm². Experimentalresults to date indicate that in some embodiments the electrocatalystloading is preferably less than about 1 mg/cm², and more preferablybetween about 0.1 and 1 mg/cm².

To promote contact between the collector, electrode, electrocatalyst,and membrane, the layers are usually compressed at high temperature. Thehousings of the individual fuel cells are configured in such a way thata good gas supply is ensured, and at the same time the product water canbe discharged properly. Typically, several fuel cells are joined to formstacks, so that the total power output is increased to economicallyfeasible levels.

In general, the electrocatalyst compositions and fuel cell electrodes ofthe present invention may be used to electrocatalyze any fuel containinghydrogen (e.g., hydrogen and reformated-hydrogen fuels). Also,hydrocarbon-based fuels may be used including saturated hydrocarbonssuch as methane (natural gas), ethane, propane and butane; garbageoff-gas; oxygenated hydrocarbons such as methanol and ethanol; andfossil fuels such as gasoline and kerosene; and mixtures thereof.

To achieve the full ion-conducting property of proton exchangemembranes, in some embodiments suitable acids (gases or liquids) aretypically added to the fuel. For example, SO₂, SO₃, sulfuric acid,trifluoromethanesulfonic acid or the fluoride thereof, also stronglyacidic carboxylic acids such as trifluoroacetic acid, and volatilephosphoric acid compounds may be used (“Ber. Bunsenges. Phys. Chem.”,Volume 98 (1994), pages 631 to 635).

Fuel Cell Uses

As set forth above, the alloy compositions of the present invention areuseful as electrocatalysts in fuel cells which generate electricalenergy to perform useful work. For example, the alloy compositions maybe used in fuel cells which are in electrical utility power generationfacilities; uninterrupted power supply devices; extraterrestrialvehicles; transportation equipment such as heavy trucks, automobiles,and motorcycles (see, Fuji et al., U.S. Pat. No. 6,048,633; Shinkai etal., U.S. Pat. No. 6,187,468; Fuji et al., U.S. Pat. No. 6,225,011; andTanaka et al., U.S. Pat. No. 6,294,280); residential power generationsystems; mobile communications equipment such as wireless telephones,pagers, and satellite phones (see, Prat et al., U.S. Pat. No. 6,127,058and Kelley et al., U.S. Pat. No. 6,268,077); mobile electronic devicessuch as laptop computers, personal data assistants, audio recordingand/or playback devices, digital cameras, digital video cameras, andelectronic game playing devices; military and aerospace equipment suchas global positioning satellite devices; and robots.

Example 1 Forming Electrocatalytic Alloys on Individually AddressableElectrodes

The electrocatalyst alloy compositions set forth in Tables A-C, infra,were prepared using the combinatorial techniques disclosed in Warren etal., U.S. Pat. No. 6,187,164; Wu et al., U.S. Pat. No. 6,045,671;Strasser, P., Gorer, S. and Devenney, M., Combinatorial ElectrochemicalTechniques For The Discovery of New Fuel-Cell Cathode Materials,Nayayanan, S. R., Goftesfeld, S. and Zawodzinski, T., eds., DirectMethanol Fuel Cells, Proceedings of the Electrochemical Society, NewJersey, 2001, p. 191; and Strasser, P., Gorer, S. and Devenney, M.,Combinational Electrochemical Strategies For The Discovery of NewFuel-Cell Electrode Materials, Proceedings of the InternationalSymposium on Fuel Cells for Vehicles, 41st Battery Symposium, TheElectrochemical Society of Japan, Nagoya 2000, p. 153. For example, anarray of independent electrodes (with areas of between about 1 and 3mm²) may be fabricated on inert substrates (e.g., glass, quartz,sapphire alumina, plastics, and thermally treated silicon). Theindividual electrodes were located substantially in the center of thesubstrate, and were connected to contact pads around the periphery ofthe substrate with wires. The electrodes, associated wires, and contactpads were fabricated from a conducting material (e.g., titanium, gold,silver, platinum, copper or other commonly used electrode materials).

Specifically, the alloy compositions set forth in Tables A-C wereprepared using a photolithography/RF magnetron sputtering technique (GHzrange) to deposit thin-film alloys on arrays of 64 individuallyaddressable electrodes. A quartz insulating substrate was provided andphotolithographic techniques were used to design and fabricate theelectrode patterns on it. By applying a predetermined amount ofphotoresist to the substrate, photolyzing preselected regions of thephotoresist, removing those regions that have been photolyzed (e.g., byusing an appropriate developer), depositing a layer of titanium about500 nm thick using RF magnetron sputtering over the entire surface andremoving predetermined regions of the deposited titanium (e.g. bydissolving the underlying photoresist), intricate patterns ofindividually addressable electrodes were fabricated on the substrate.

Referring to FIG. 3, the fabricated array 20 consisted of 64individually addressable electrodes 21 (about 1.7 mm in diameter)arranged in an 8×8 square that were insulated from each other (byadequate spacing) and from the substrate 24 (fabricated on an insulatingsubstrate), and whose interconnects 22 and contact pads 23 wereinsulated from the electrochemical testing solution (by the hardenedphotoresist or other suitable insulating material).

After the initial array fabrication and prior to depositing theelectrocatalyst alloys for screening, a patterned insulating layercovering the wires and an inner portion of the peripheral contact pads,but leaving the electrodes and the outer portion of the peripheralcontact pads exposed (preferably approximately half of the contact padis covered with this insulating layer) was deposited. Because of theinsulating layer, it is possible to connect a lead (e.g., a pogo pin oran alligator clip) to the outer portion of a given contact pad andaddress its associated electrode while the array is immersed insolution, without having to worry about reactions that can occur on thewires or peripheral contact pads. The insulating layer was a hardenedphotoresist, but any other suitable material known to be insulating innature could have been used (e.g., glass silica, alumina, magnesiumoxide, silicon nitride, boron nitride, yttrium oxide, or titaniumdioxide).

Following the creation of the titanium coated array, a steel mask having64 holes (1.7 mm in diameter) was pressed onto the substrate to preventdeposition of sputtered material onto the insulating resist layer. Thedeposition of the electrocatalyst alloys was also accomplished using RFmagnetron sputtering and a two shutter masking system as described by Wuet al. which enable the deposition of material onto 1 or more electrodesat a time. Each individual thin-film electrocatalyst alloy is created bya super lattice deposition method. For example, when preparing a ternaryalloy electrocatalyst composition, metals M1, M2 and M3 are to bedeposited and alloyed onto one electrode. First, a metal M1 sputtertarget is selected and a thin film of M1 having a defined thickness isdeposited on the electrode. This initial thickness is typically fromabout 3 to about 12 Å. After this, metal M2 is selected as the sputtertarget and a layer of M2 is deposited onto the layer of M1. Thethickness of M2 layer is also from about 3 to about 12 Å. Thethicknesses of the deposited layers are in the range of the diffusionlength of the metal atoms (e.g., about 10 to about 30 Å) which allowsin-situ alloying of the metals. Then, a layer of M3 is deposited ontothe M1-M2 alloy forming a M1-M2-M3 alloy film. As a result of the threedeposition steps, an alloy thin-film (9-25 Å thickness) of the desiredstoichiometry is created. This concludes one deposition cycle. In orderto achieve the desired total thickness of a cathode electrocatalystsmaterial, deposition cycles are repeated as necessary which results inthe creation of a super lattice structure of a defined total thickness(typically about 700 Å). Although the number, thickness (stoichiometry)and order of application of the individual metal layers may bedetermined manually, it is desirable to utilize a computer program todesign an output file which contains the information necessary tocontrol the operation of the sputtering device during the preparation ofa particular library wafer (i.e., array). One such computer program isthe LIBRARY STUDIO software available from Symyx Technologies, Inc. ofSanta Clara, Calif. and described in European Patent No. 1080435 B1. Thecompositions of several as sputtered alloy compositions were analyzedusing x-ray fluorescence (XRF) to confirm that they were consistent withdesired compositions (chemical compositions determined using x-rayfluorescence are within about 5% of the actual composition).

Arrays were prepared to evaluate the specific alloy compositions setforth in Tables A-C below. On each array one electrode consistedessentially of platinum and it served as an internal standard for thescreening operation.

TABLE A End Current End Current Relative Density Density/ Activity(Absolute Weight Compared Electrode activity) Fraction of to Internal PtZn Ni Number mA/cm2 Pt Pt atomic % atomic % atomic % 15 −0.821 −1.5533.921 25.49 8.65 65.86 9 −0.582 −1.097 2.782 26.41 35.25 38.34 1 −0.552−1.011 2.635 27.51 31.95 40.54 3 −0.533 −1.053 2.548 24.33 27.25 48.4211 −0.489 −1.015 2.334 22.60 30.68 46.72 10 −0.473 −0.929 2.259 24.6933.19 42.12 36 −0.389 −0.718 1.860 27.40 37.20 35.40 17 −0.369 −0.7211.763 25.07 39.30 35.63 18 −0.327 −0.675 1.562 22.98 37.43 39.59 43−0.325 −0.595 1.555 28.10 47.69 24.21 12 −0.309 −0.696 1.477 19.99 27.5552.45 44 −0.302 −0.466 1.442 37.15 38.45 24.40 45 −0.297 −0.407 1.41846.34 29.06 24.59 37 −0.294 −0.457 1.404 36.22 28.11 35.67 46 −0.285−0.357 1.364 55.68 19.53 24.79 54 −0.271 −0.314 1.294 66.94 20.22 12.8353 −0.269 −0.335 1.286 57.19 30.09 12.73 51 −0.257 −0.393 1.226 38.1449.34 12.52 55 −0.250 −0.273 1.192 76.86 10.20 12.94 52 −0.243 −0.3311.161 47.59 39.79 12.62 38 −0.225 −0.310 1.075 45.17 18.88 35.95 47−0.221 −0.258 1.056 65.17 9.84 24.99 26 −0.216 −0.480 1.032 20.76 42.9236.32 30 −0.211 −0.331 1.009 35.33 18.28 46.40 64 −0.209 −0.209 1.000100.00 0.00 0.00 29 −0.206 −0.383 0.984 26.73 27.22 46.05 39 −0.206−0.259 0.982 54.26 9.52 36.23 2 −0.202 −0.382 0.965 26.06 29.81 44.13 31−0.198 −0.274 0.944 44.05 9.21 46.74 23 −0.196 −0.310 0.937 34.48 8.9256.60 22 −0.187 −0.351 0.895 26.09 17.71 56.19 50 −0.178 −0.323 0.85128.84 58.73 12.42 4 −0.163 −0.340 0.777 22.23 24.14 53.62 19 −0.127−0.284 0.607 20.37 35.09 44.54 13 −0.112 −0.286 0.535 16.65 23.56 59.795 −0.096 −0.217 0.458 19.63 20.29 60.08 25 −0.078 −0.159 0.371 23.3744.42 32.21 6 −0.051 −0.130 0.241 16.31 15.38 68.31 20 −0.035 −0.0880.167 17.01 32.09 50.90 41 −0.016 −0.039 0.075 18.19 60.04 21.77 27−0.013 −0.032 0.061 17.39 40.99 41.62 34 −0.011 −0.026 0.051 17.78 50.3031.92 28 −0.003 −0.011 0.017 12.86 38.40 48.74 21 0.000 0.000 −0.00112.54 28.08 59.39 14 0.001 0.004 −0.006 12.23 18.26 69.51 7 0.002 0.006−0.008 11.93 8.91 79.16 49 0.003 0.010 −0.015 13.94 72.85 13.21 42 0.0030.010 −0.016 13.56 60.74 25.70 35 0.004 0.012 −0.018 13.20 49.28 37.52

TABLE B End Current End Current Density Density/ Relative (AbsoluteWeight Activity Electrode activity) Fraction of Compared Pt Zn Fe NumbermA/cm2 Pt Internal Pt atomic % atomic % atomic % 45 −1.719 −2.444 7.85841.75 19.36 38.88 52 −1.543 −2.211 7.053 40.43 9.37 50.20 38 −1.348−1.901 6.160 43.17 30.03 26.80 31 −0.583 −0.816 2.666 44.68 41.45 13.8751 −0.539 −0.913 2.463 29.65 9.14 61.20 23 −0.471 −0.770 2.153 33.8552.18 13.97 37 −0.433 −0.721 1.981 31.61 29.24 39.14 17 −0.426 −1.1371.949 15.40 29.44 55.16 25 −0.425 −1.235 1.944 13.65 25.83 60.52 36−0.423 −0.918 1.935 20.65 28.50 50.86 44 −0.417 −0.700 1.906 30.60 18.8750.53 30 −0.377 −0.621 1.722 32.69 40.32 26.99 29 −0.369 −0.793 1.68921.33 39.26 39.41 2 −0.331 −0.824 1.513 17.19 38.57 44.25 53 −0.323−0.412 1.475 51.76 9.62 38.62 47 −0.306 −0.356 1.398 65.90 20.43 13.6743 −0.306 −0.669 1.397 20.00 18.41 61.59 33 −0.300 −0.988 1.372 11.5221.46 67.02 54 −0.288 −0.337 1.315 63.70 9.87 26.43 46 −0.287 −0.3641.312 53.50 19.88 26.61 55 −0.285 −0.312 1.302 76.28 10.14 13.58 9−0.279 −0.699 1.276 16.87 32.45 50.68 39 −0.267 −0.336 1.220 55.37 30.8613.77 50 −0.258 −0.571 1.181 19.40 8.92 71.68 10 −0.231 −0.611 1.05615.72 36.04 48.24 64 −0.219 −0.219 1.000 100.00 0.00 0.00 26 −0.215−0.700 0.981 11.80 29.32 58.88 11 −0.185 −0.528 0.844 14.27 40.52 45.2018 −0.163 −0.469 0.744 13.95 33.02 53.03 3 −0.153 −0.402 0.699 16.0442.92 41.04 27 −0.151 −0.597 0.692 9.38 33.90 56.72 19 −0.116 −0.3750.531 12.10 37.59 50.31 1 −0.078 −0.185 0.355 18.12 35.01 46.87 42−0.060 −0.355 0.274 5.68 18.81 75.52 22 −0.057 −0.121 0.260 22.06 50.7627.18 49 −0.043 −0.259 0.197 5.50 9.11 85.39 15 −0.025 −0.052 0.11322.85 63.08 14.07 12 −0.024 −0.076 0.109 12.42 46.28 41.30 28 −0.015−0.089 0.070 6.06 40.17 53.77 41 −0.014 −0.055 0.063 8.87 16.03 75.10 35−0.009 −0.055 0.043 5.86 29.14 65.00

TABLE C End Current End Current Relative Density Density/ Activity(Absolute Weight Compared Electrode activity) Fraction of to Internal PtZn Fe Number mA/cm2 Pt Pt atomic % atomic % atomic % 40 −1.468 −2.6523.635 28.14 43.23 28.62 38 −1.206 −1.847 2.986 36.58 26.22 37.20 39−0.698 −1.164 1.728 31.81 35.83 32.35 31 −0.536 −0.745 1.328 44.32 26.9828.70 37 −0.525 −0.731 1.299 43.02 13.22 43.76 16 −0.509 −0.568 1.26173.20 14.67 12.14 24 −0.475 −0.611 1.177 52.52 27.77 19.70 30 −0.470−0.598 1.163 52.40 13.67 33.93 32 −0.452 −0.681 1.120 38.40 36.73 24.8745 −0.446 −0.774 1.104 29.26 25.51 45.24 8 −0.404 −0.404 1.000 100.000.00 0.00 44 −0.374 −0.586 0.926 34.25 12.80 52.95 23 −0.157 −0.1860.390 62.43 14.15 23.42 51 −0.109 −0.201 0.271 26.01 12.40 61.58 52−0.098 −0.200 0.242 22.32 24.83 52.85 46 −0.089 −0.169 0.219 25.53 34.9839.48 59 −0.051 −0.132 0.126 15.75 24.19 60.06 53 −0.043 −0.097 0.10619.55 34.17 46.28 54 −0.035 −0.087 0.087 17.39 41.45 41.16 55 −0.029−0.079 0.073 15.66 47.28 37.07 48 −0.024 −0.053 0.059 20.36 48.17 31.4747 −0.023 −0.048 0.057 22.65 42.32 35.03 56 −0.009 −0.027 0.023 14.2452.05 33.71 61 −0.008 −0.025 0.019 12.34 40.61 47.05 58 −0.007 −0.0170.019 18.27 12.03 69.70 60 0.000 −0.001 0.001 13.84 33.40 52.77 62 0.0000.001 −0.001 11.13 46.41 42.46 64 0.001 0.004 −0.002 9.31 55.17 35.51 630.001 0.004 −0.003 10.14 51.18 38.68

Example 2 Screening Alloys for Electrocatalytic Activity

The alloy compositions set forth in Tables A-C (set forth above) thatwere synthesized on arrays according to the method set forth in Example1 were screened according to one and/or two protocols (set forth below)for electrochemical reduction of molecular oxygen to water to determinerelative electrocatalytic activity against the internal and/or externalplatinum standard. The first protocol was used to screen the alloycompositions set forth in Tables A and B, and the second protocol wasused to screen the alloy compositions in Table C. The two protocolsdiffer, in part, in the choice of electrolyte: the first protocol wasperformed with an aqueous 1 M HClO₄ solution; and the second protocolwas performed with an aqueous 0.5 M H₂SO₄ solution. The second protocolwas developed to better reflect the conditions under which a catalystwould be subjected to in a PEM fuel cell and to more accurately evaluatethe catalyst compositions.

In general, the array wafers were assembled into an electrochemicalscreening cell and a screening device established an electrical contactbetween the 64 electrode electrocatalysts (working electrodes) and a64-channel multi channel potentiostat used for the screening.Specifically, each wafer array was placed into a screening device suchthat all 64 spots are facing upward and a tube cell body that wasgenerally annular and having an inner diameter of about 2 inches (5 cm)was pressed onto the upward facing wafer surface. The diameter of thistubular cell was such that the portion of the wafer with the squareelectrode array formed the base of a cylindrical volume while thecontact pads were outside the cylindrical volume. A liquid ionicsolution (electrolyte) was poured into this cylindrical volume and acommon counter electrode (i.e., platinum gauze), as well as a commonreference electrode (e.g., mercury/mercury sulfate reference electrode(MMS)), were placed into the electrolyte solution to close theelectrical circuit.

First Protocol

During the first protocol a rotator shaft with blades was also placedinto the electrolyte to provide forced convection-diffusion conditionsduring the screening. The rotation rate was typically between about 300to about 400 rpm. Depending on the screening experiment either argon orpure oxygen was bubbled through the electrolyte during the measurements.Argon served to remove O₂ gas in the electrolyte to simulate O₂-freeconditions used for the initial conditioning of the electrocatalysts.The introduction of pure oxygen served to saturate the electrolyte withoxygen for the oxygen reduction reaction. During the screening, theelectrolyte was maintained at 60° C. and the rotation rate was constant.Three groups of tests were performed to screen the activity of theelectrocatalysts. The electrolyte (1 M HClO₄) was purged with argon forabout 20 minutes prior to the electrochemical measurements. The firstgroup of tests comprised cyclic voltammetric measurements while purgingthe electrolyte with argon. Specifically, the first group of testscomprised:

-   -   a. a potential sweep from about OCP to about +0.3 V to about        −0.63 V and back to about +0.3 V at a rate of about 20 mV/s;    -   b. twelve consecutive potential sweeps from OCP to about +0.3 V        to about −0.7 V and back to about +0.3 V at a rate of about 200        mV/s; and    -   c. a potential sweep from about OCP to about +0.3 V to about        −0.63 V and back to about +0.3 V at a rate of about 20 mV/s.        The electrolyte was then purged with oxygen for approximately 30        minutes. The following second group of tests were performed        while continuing to purge with oxygen:    -   a. measuring the open circuit potential (OCP) for a minute;        then, starting at OCP the voltage was swept down to about −0.4 V        at a rate of about 10 mV/s;    -   b. measuring the OCP for a minute; then applying a potential        step from OCP to about +0.1 V while measuring the current for        about 5 minutes; and    -   c. measuring the OCP for a minute; then applying a potential        step from OCP to about +0.2 V while monitoring the current for        about 5 minutes.        The third group of tests comprised a repeat of the second group        of tests after about one hour from completion of the second        group of tests. The electrolyte was continually stirred and        purged with oxygen during the waiting period. All the foregoing        test voltages are with reference to a mercury/mercury sulfate        (MMS) electrode. Additionally, an external platinum standard        comprising an array of 64 platinum electrodes in which the        oxygen reduction activity of the 64 platinum electrodes averaged        −0.35 mA/cm² at +0.1V vs. a mercury/mercury sulfate electrode        was used to monitor the tests to ensure the accuracy of the        oxygen reduction evaluation.

Second Protocol

The second protocol was identical to the first protocol except anaqueous 0.5 M H₂SO₄ solution was used and procedures were implemented toensure that the internal platinum standard electrode was stabilized bythe first group of tests prior to performing the second and third groupsof tests. Specifically, after test c of the first group of tests wascompleted, the shape of the cyclic voltammetric (CV) profile of theinternal platinum standard catalyst obtained in test c was compared toan external standard CV profile obtained from a platinum thin-filmelectrode that had been pretreated until a stable CV was obtained. Ifthe test c resulted in a cyclic voltammogram similar to the externalstandard, the first group of experiments was considered complete. If theshape of the internal platinum standard cyclic voltammogram of test cdid not result in a behavior similar to the external standard platinumcyclic voltammogram, the tests b and c of the first group were repeateduntil the internal platinum standard catalyst showed the desiredstandard voltammetric profile. The process ensured that the internalplatinum standard catalyst showed a stable and well-defined oxygenreduction activity in subsequent experiments. The electrolyte was thenpurged with oxygen for approximately 30 minutes and the second and thirdgroup of tests were performed.

The specific alloy compositions set forth in Tables A-C were preparedand screened in accordance with the above-described methods and theresults are set forth therein. The screening results in Tables A-C arefor the third test group steady state currents at +0.1 V MMS. Thecurrent value reported (End Current Density) is the result of averagingthe last three current values of the chronoamperometric test normalizedfor geometric surface area. Referring to Table A, the Pt—Zn—Ni alloycompositions corresponding to Electrode Numbers 15, 9, 1, 3, 11, 10, 36,17, 18, 43, 12, 44, 45, 37, 46, 54, 53, 51, 55, 52, 38, 47, 26, and 30exhibited an oxygen reduction activity greater than the internalplatinum standard. Referring to Table B, the Pt—Zn—Fe alloy compositionscorresponding to Electrode Numbers 40, 38, 39, 31, 37, 16, 24, 30, 32,and 45 exhibited an oxygen reduction activity greater than the internalplatinum standard. Referring to Table C, the Pt—Zn—Fe alloy compositionscorresponding to Electrode Numbers 45, 52, 38, 31, 51, 23, 37, 17, 25,36, 44, 30, 29, 2, 53, 47, 43, 33, 54, 46, 55, 9, 39, 50, and 10exhibited an oxygen reduction activity greater than the internalplatinum standard.

Example 3 Synthesis of Supported Electrocatalyst Alloys

The synthesis of Pt₁₉Zn₂₄Ni₅₇, Pt₂₇Zn₃₅Ni₃₈, Pt₃₆Zn₃₀Fe₃₄, Pt₄₂Zn₁₉Fe₃₉,Pt₄₉Zn₁₈Fe₃₃, and Pt₂₃Zn₄₂Fe₃₅ alloys (see, Table D, Target CatalystComp., infra) on carbon support particles was attempted according todifferent process conditions in order to evaluate the performance of thealloys while in a state that is typically used in fuel cell. To do so,the alloy component precursors were deposited or precipitated onsupported platinum powder (i.e., platinum nanoparticles supported oncarbon black particles). Platinum supported on carbon black iscommercially available from companies such as Johnson Mafthey, Inc., ofNew Jersey and E-Tek Div. of De-Nora, N.A., Inc., of Sommerset, N.J.Such supported platinum powder is available with a wide range ofplatinum loading. The supported platinum powder used in this example hada nominal platinum loading of about 40 percent by weight, a platinumsurface area of between about 150 and about 170 m²/g (determined by COadsorption), a combined carbon and platinum surface area between about350 and about 400 m²/g (determined by N₂ adsorption), and an averageparticle size of less than about 0.5 mm (determined by sizing screen).

Referring to Table D, the electrocatalyst alloys corresponding to thetarget compositions of Pt₁₉Zn₂₄Ni₅₇, Pt₂₇Zn₃₅Ni₃₈, Pt₃₆Zn₃₀Fe₃₄, andPt₄₂Zn₁₉Fe₃₉ (HFC 16, 17, 26, 27, 32 and 33) were formed on carbonsupport particles using a chemical precipitation method according to thefollowing steps. First, about 0.5 g of carbon supported platinum powder(36.4 wt % Pt) was dispersed in about 200 mL of room temperature 18 MΩdeionized water using an ultrasonic blending device (e.g., an AQUASONIC50 D set at power level 9) for about 2 hours to form a slurry. Theslurry was stirred using a magnetic stirring device, and while beingstirred, appropriate volumes based on the targeted electrocatalystcomposition of one or more appropriate solutions comprising the metalsto be alloyed with the platinum nanoparticles were added drop-wise tothe slurry (i.e., a 1M Ni(NO₃)₂.6H₂O, 1M Fe(NO₃)₃.9H₂O and 1 MZn(NO₃)₂.6H₂O aqueous solutions). Specifically, to produce HFC16 and 17,2.56 ml of the 1M Ni(NO₃)₂.6H₂O solution and 1.23 ml of the 1MZn(NO₃)₂.6H₂O solution were added; to produce HFC 26 and 27, 0.4 mL ofthe 1M Zn(NO₃)₂.6H₂O solution and 0.45 ml of the 1M Fe(NO₃)₃.9H₂Osolution were added; to produce HFC 32, 0.22 ml of the 1M Zn(NO₃)₂.6H₂Osolution and 0.45 ml of the 01M Fe(NO₃)₃.9H₂O solution were added; toproduce HFC 33, 0.71 ml of the 1M Ni(NO₃)₂.6H₂O solution and 0.65 ml ofthe 1M Zn(NO₃)₂.6H₂O solution were added. The stirring was continued andthe slurry containing the dissolved metal salts was heated to atemperature between about 60 and about 90° C. for about 1 hour.Precipitation of compounds comprising the metals was then initiated byslowly adding a 10 wt % ammonium hydroxide aqueous solution to theslurry until the slurry had a pH of about 10. The slurry was stirred forabout 15 more minutes. The slurry was then filtered from the liquidunder vacuum after which the filtrate was washed with about 150 mL ofdeionized water. The powder was then dried at a temperature betweenabout 60 and about 100° C. for about 4 hours to about 8 hours.

Referring to Table D, the electrocatalyst alloys corresponding to thetarget compositions Pt₄₉Zn₁₈Fe₃₃, Pt₄₂Zn₁₉Fe₃₉, and Pt₂₃Zn₄₂Fe₃₅ wereformed on carbon support particles using a freeze-drying precipitationmethod. The freeze-drying method comprised forming a precursor solutioncomprising the desired metal atoms in the desired concentrations. Forexample, to prepare the target Pt₄₉Zn₁₈Fe₃₃ alloy composition having afinal nominal platinum loading of about 18 percent by weight (HFC 146),0.054 g of Fe(NO₃)₃.9H₂O was dissolved in about 5 ml H₂O then 0.022 g ofZn(NO₃)₂.6H₂O was dissolved in the solution. To prepare the targetPt₄₂Zn₁₉Fe₃₉ alloy compositions having a final nominal platinum loadingof about 32 percent by weight (HFC 150 and 151), 0.143 g ofFe(NO₃)₃.9H₂O was dissolved in about 5 ml H₂O and then 0.051 g ofZn(NO₃)₂.6H₂O was dissolved in the solution. To prepare the targetPt₄₂Zn₁₉Fe₃₉ alloy compositions having a final nominal platinum loadingof about 32 percent by weight (HFC 163-165), 0.285 g of Fe(NO₃)₃.9H₂Owas dissolved in about 7.5 ml H₂O and then 0.102 g of Zn(NO₃)₂.6H₂O wasdissolved in the solution. To prepare the target Pt₂₃Zn₄₂Fe₃₅ alloycompositions (HFC 215 and 216), 0.234 g of Fe(NO₃)₃.9H₂O was dissolvedin about 5 ml H₂O and then 0.207 g of Zn(NO₃)₂.6H₂O was dissolved in thesolution.

Referring to Table D, the HFC 146 solution was introduced into separateHDPE (High Density Poly Ethylene) vials containing about 0.200 g ofsupported platinum powder which had a final nominal platinum loading ofabout 19.2 percent by weight resulting in a black suspension. The HFC150, 151, 215, and 216 solutions were introduced into separate HDPE(High Density Poly Ethylene) vials containing about 0.200 g of supportedplatinum powder which had a final nominal platinum loading of about 37.1percent by weight resulting in a black suspension. The HFC 163-165solutions were introduced into separate HDPE (High Density PolyEthylene) vials containing about 0.400 g of supported platinum powderwhich had a final nominal platinum loading of about 37.1 percent byweight resulting in a black suspension. The suspensions were homogenizedby immersing a probe of a BRANSON SONIFIER 150 into the vial andsonicating the mixture for about 1 minute at a power level of 3.

The vials containing the homogenized suspensions were then immersed in aliquid nitrogen bath for about 3 minutes to solidify the suspensions.The solid suspensions were then freeze-dried for about 24 hours using aLABONCO FREEZE DRY SYSTEM (Model 79480) to remove the solvent. The trayand the collection coil of the freeze dryer were maintained at about 26°C. and about −48° C., respectively, while evacuating the system (thepressure was maintained at about 0.15 mbar). After freeze-drying, eachvial contained a powder comprising the supported platinum powder, andzinc, and nickel or iron precursors deposited thereon.

The recovered precursor powders (chemically precipitated andfreeze-dried) were then subjected to a heat treatment to reduce theprecursors to their metallic state, and to alloy the metals with eachother and the platinum on the carbon black particles. One particularheat treatment comprised heating the powder in a quartz flow furnacewith an atmosphere comprising about 6% H₂ and 94% Ar using a temperatureprofile of room temperature to about 40° C. at a rate of about 5°C./min; holding at about 40° C. for 2 hours; increasing the temperatureto about 200° C. at a rate of 5° C./min; holding at about 200° C. fortwo hours; increasing the temperature at a rate of about 5° C./min toabout 600, 700 or 900° C.; holding at a max temperature of about 600,700 or 900° C. for a duration of about one, two, three, seven or eighthours (indicated in Table D); and cooling down to room temperature.

In order to determine the actual composition of the supportedelectrocatalyst alloys, the differently prepared alloys (e.g., bycomposition variation or by heat treatment variation) were subjected toICP elemental analysis or subjected to EDS (Electron DispersiveSpectroscopy) elemental analysis. For the latter technique, the samplepowders were compressed into 6 mm diameter pellets with a thickness ofabout 1 mm. The target alloy composition and actual composition for theprepared supported electrocatalyst alloys are also set forth in Table D.

TABLE D Catalyst Max Log Pt Pt Mass Mass Alloying Target Measured MassActivity at Relative Activity Target Temp for Actual Pt Pt Activity+0.15 V performance at 0.15 V Powder Catalyst a duration CatalystLoading Loading at +0.15 V MMS at +0.15 V MMS Name Comp. (° C./hrs)Comp. (wt %) (wt %) MMS (mA/mg Pt) MMS (mA/mg) HFC 10 Pt — Pt 37.9 37.92.11 128.82 1.00 48.82 by ICP HFC 10 Pt — Pt 37.9 37.9 2.26 181.97 1.4168.97 tested at by ICP 60° C. HFC 16 Pt₁₉Zn₂₄Ni₅₇ 700/2 Pt₂₇Zn₂₀Ni₅₃25.0 27.2 2.36 230.60 1.79 62.61 by EDS HFC 17 Pt₁₉Zn₂₄Ni₅₇ 900/2 — 25.0— — — — 40.52 HFC 33 Pt₂₇Zn₃₅Ni₃₈ 700/8 Pt₃₃Zn₂₃Ni₄₄ 32.0 30.0 2.45280.56 2.18 84.17 by ICP HFC 26 Pt₃₆Zn₃₀Fe₃₄ 700/3 — 30.0 — — — — 94.65HFC 27 Pt₃₆Zn₃₀Fe₃₄ 700/8 Pt₄₉Zn₁₈Fe₃₃ 30.0 32.9 2.51 323.54 2.51 106.44by EDS HFC 27 Pt₃₆Zn₃₀Fe₃₄ 700/8 Pt₄₈Zn₂₀Fe₃₂ 30.0 32.9 2.84 691.71 3.80227.57 tested at by EDS 60° C. HFC 32 Pt₄₂Zn₁₉Fe₃₉ 700/8 Pt₅₁Zn₁₃Fe₃₆34.0 36.0 2.46 285.22 2.21 102.68 by ICP HFC 146 Pt₄₉Zn₁₈Fe₃₃ 700/7Pt₅₁Zn₂₀Fe₂₉ 18.0 17.8 2.48 298.66 2.32 53.16 by EDS HFC 150Pt₄₂Zn₁₉Fe₃₉ 700/7 Pt₄₂Zn₂₀Fe₃₈ 32.0 31.5 2.42 260.79 2.02 82.15 by EDSHFC 151 Pt₄₂Zn₁₉Fe₃₉ 900/2 — 32.0 — — — — 64.28 HFC 163 Pt₄₂Zn₁₉Fe₃₉700/2 Pt₄₂Zn₂₂Fe₃₆ 32.0 30.7 2.55 352.90 2.74 108.34 by EDS HFC 164Pt₄₂Zn₁₉Fe₃₉ 700/1 Pt₄₂Zn₁₉Fe₃₉ 32.0 28.5 2.45 282.93 2.20 80.64 by EDSHFC 165 Pt₄₂Zn₁₉Fe₃₉ 600/7 Pt₄₂Zn₂₀Fe₃₈ 32.0 30.7 2.51 321.18 2.49 98.60by EDS HFC 215 Pt₂₃Zn₄₂Fe₃₅ 700/7 Pt₃₃Zn₁₂Fe₅₅ 26.6 30.9 2.23 169.411.32 52.38 by EDS HFC 216 Pt₂₃Zn₄₂Fe₃₅ 900/2 — 26.6 — — — — 23.72

Example 4 Evaluating the Electrocatalytic Activity of SupportedElectrocatalysts

The supported alloy electrocatalysts set forth in Table D and formedaccording to Example 3 were subjected to electrochemical measurements toevaluate their activities. For the evaluation, the supported alloyelectrocatalysts were applied to a rotating disk electrode (RDE) as iscommonly used in the art (see, Rotating disk electrode measurements onthe CO tolerance of a high-surface area Pt/Vulcan carbon fuel cellelectrocatalyst, Schmidt et al., Journal of the Electrochemical Society(1999), 146(4), 1296-1304; and Characterization of high-surface-areaelectrocatalysts using a rotating disk electrode configuration, Schmidtet al., Journal of the Electrochemical Society (1998), 145(7),2354-2358). Rotating disk electrodes are a relatively fast and simplescreening tool for evaluating supported electrocatalysts with respect totheir intrinsic electrolytic activity for oxygen reduction (e.g., thecathodic reaction of a fuel cell).

The rotating disk electrode was prepared by depositing an aqueous-basedink that comprises the support electrocatalyst and a NAFION solution ona glassy carbon disk. The concentration of electrocatalyst powder in theNAFION solution was about 1 mg/mL. The NAFION solution comprised theperfluorinated ion-exchange resin, lower aliphatic alcohols and water,wherein the concentration of resin is about 5 percent by weight. TheNAFION solution is commercially available from the ALDRICH catalog asproduct number 27,470-4. The glassy carbon electrodes were 5 mm indiameter and were polished to a mirror finish. Glassy carbon electrodesare commercially available, for example, from Pine Instrument Company ofGrove City, Pa. An aliquot of 10 μL electrocatalyst suspension was addedto the carbon substrate and allowed to dry at a temperature betweenabout 60 and 70° C. The resulting layer of NAFION and electrocatalystwas less than about 0.2 μm thick. This method produced slightlydifferent platinum loadings for each electrode made with a particularsuspension, but the variation was determined to be less than about 10percent by weight.

After being dried, the rotating disk electrode was immersed into anelectrochemical cell comprising an aqueous 0.5 M H₂SO₄ electrolytesolution maintained at room temperature or 60° C. (as indicated in TableD). The 60° C. electrolyte temperature used to evaluate HFC 10 (Ptstandard) and HFC 27 was selected to more closely resemble to theconditions under which the tests of Example 2 were performed and/oractual fuel cell conditions. Before performing any measurements, theelectrochemical cell was purged of oxygen by bubbling argon through theelectrolyte for about 20 minutes. All measurements were taken whilerotating the electrode at about 2000 rpm, and the measured currentdensities were normalized either to the glassy carbon substrate area orto the platinum loading on the electrode. Two groups of tests wereperformed to screen the activity of the supported electrocatalysts. Thefirst group of tests comprised cyclic voltammetric measurements whilepurging the electrolyte with argon. Specifically, the first groupcomprised:

-   -   a. two consecutive potential sweeps starting from OCP to about        +0.35V then to about −0.65V and back to OCP at a rate of about        50 mV/s;    -   b. two hundred consecutive potential sweeps starting from OCP to        about +0.35V then to about −0.65V and back to OCP at a rate of        about 200 mV/s; and    -   c. two consecutive potential sweeps starting from OCP to about        +0.35V then to about −0.65V and back to OCP at a rate of about        50 mV/s.        The second test comprised purging with oxygen for about 15        minutes followed by a potential sweep test for oxygen reduction        while continuing to purge the electrolyte with oxygen.        Specifically, potential sweeps from about −0.45 V to +0.35 V        were performed at a rate of about 5 mV/s to evaluate the initial        activity of the electrocatalyst as a function of potential and        to create a geometric current density plot. The electrocatalysts        were evaluated by comparing the diffusion corrected activity at        0.15 V. All the foregoing test voltages are with reference to a        mercury/mercury sulfate electrode. Also, it is to be noted that        the oxygen reduction measurements for a glassy carbon RDE        without an electrocatalyst did not show any appreciable activity        within the potential window.

The above-described supported electrocatalyst alloy compositions wereevaluated in accordance with the above-described method and the resultsare set forth in Table D. The alloy compositions Pt₂₇Zn₂₀Ni₅₃,Pt₃₃Zn₂₃Ni₄₄, Pt₄₉Zn₁₈Fe₃₃, Pt₄₈Zn₂₀Fe₃₂, Pt₅₁Zn₁₃Fe₃₆, Pt₅₁Zn₂₀Fe₂₉,Pt₄₂Zn₂₀Fe₃₈, Pt₄₂Zn₂₂Fe₃₆, Pt₄₂Zn₁₉Fe₃₉, Pt₄₂Zn₂₀Fe₃₈, and Pt₃₃Zn₁₂Fe₅₅exhibited oxygen reduction activities greater than that of carbonsupported platinum. The results of the evaluation also indicate, amongother things, that it may take numerous iterations to develop a set ofparameters for producing the target alloy composition. Also evidenced bythe data, is that activity can be adjusted by changes in the processingconditions and fuel cell operation conditions. For example, increasingthe temperature of the electrolyte from room temperature to about 60° C.for the Pt standard (HFC 10) and the Pt₄₈Zn₂₀Fe₃₂ alloy (HFC 27)resulted in significantly greater activity increase for the Pt₄₈Zn₂₀Fe₃₂than for the platinum standard.

Further, without being held to a particular theory, it is presentlybelieved that differences in activity for similar alloy compositions maybe due to several factors such as alloy homogeneity (e.g., an alloy, asdefined below, may have regions in which the constituent atoms show apresence or lack of order, i.e., regions of solid solution within anordered lattice, or some such superstructure), changes in the latticeparameter due to changes in the average size of component atoms, changesin particle size, and changes in crystallographic structure/symmetry.The ramifications of structure and symmetry changes are often difficultto predict. Platinum and nickel have a face-centered cubic (fcc)structure whereas iron often crystallizes in a body-centered cubic (bcc)structure however fcc is known while the structure of zinc is adistorted form of hexagonal closest packing. Pt—Fe alloys crystallize instructures from primitive cubic to primitive tetragonal, dependent oncomposition; Pt—Ni alloys may form complete solid solutions based on thestructural similarity of the end members; and Pt—Zn, Fe—Zn and Ni—Znalloys may be significantly more complex. Clearly, symmetry variationsare to be expected across a multi-component compositional range. Forexample, in the Pt—Fe system, as the amount of iron added to platinumincreases, the lattice of the resulting alloy may be expected to changefrom an fcc lattice to a tetragonal primitive lattice. Morespecifically, the possibility exists that as the relative ratio of metal(in this case Fe) to platinum goes from 0 to 1, an fcc-based solidsolution first occurs (e.g., Fe and Pt may mix randomly within someconcentration limits, or under some specific synthesis conditions), andout of this solid solution an ordered phase may gradually crystallize(e.g., Pt₃Fe, primitive cubic structure) only to return to a solidsolution (disordered alloy) and again back to an ordered phase (now witheither a primitive or face-centered tetragonal structure) as the formulaPtFe is achieved. Symmetry changes (e.g., those associated with thechanges from a cubic face-centered structure to a primitive tetragonalstructure) may result in significant changes in the x-ray diffractionpattern. These changes may also be accompanied by more subtle changes inlattice parameters that may be indicative of the resulting changes inthe size of the respective metal constituents. For example, the12-coordinate metallic radii of platinum, nickel, iron, and zinc are1.39 Å, 1.25 Å, 1.26 Å, and 1.37 Å respectively, and as metals aresubstituted for platinum, the average metal radius, and consequently theobserved lattice parameter of a disordered alloy may be expected tocontract or expand accordingly. Thus, in the case of disordered alloys,the average radius may be used as an indicator of lattice changes as afunction of stoichiometry, or alternatively, as an indicator ofstoichiometry based on observed diffraction patterns. However, while theaverage radii may be useful as a general rule, experimental results aretypically expected to conform only in a general manner because localordering, significant size disparity between atoms, significant changesin symmetry, and other factors may produce results that are inconsistentwith expectations. This may be particularly true in the case of orderedalloys where an increase in the directionality of bonding may occur.

An interpretation of XRD analyses for the foregoing supported alloys isset forth below. Interpretation of XRD analyses can be subjective, andtherefore, the following conclusions are not intended to be limiting.The predicted change in the average radius for the Pt₂₇Zn₂₀ Ni₅₃ alloys(HFC 16 and 17) was a contraction of 5.4% versus platinum. Thedetermined contraction of HFC 16 was about 2.8%, which was less thanpredicted. The smaller than anticipated amount of contraction may be theresult of an appreciable amount of Ni impurity that was observed. Thedetermined contraction of HFC 17 was about 6.0%. HFC 16 and 17 hadparticle sizes of approximately 2.3 and 4.0 nm, respectively, based onobserved diffraction peak widths. Thus, although HFC 16 has an impurity,the smaller particles resulted in improved electrochemical performance,as noted in Table D.

The predicted change in the average radius for the Pt₄₉Zn₁₈Fe₃₃ alloys(HFC 26 and 27) was a decrease of 3.1% versus platinum. The determinedcontraction of HFC 26 and 27 was about 3.2%. HFC 26 and 27 appeared todisplay the lattice parameter and structural characteristics of theordered alloy PtFe, with HFC 27 being more crystallographically ordered.Based on the stoichiometry, this may imply partial structuralreplacement of Fe by Zn. If true, the stoichiometry may be alternativelyreported as Pt(Fe_(0.65)Zn_(0.35)). HFC 26 and 27 displayed particlesizes of approximately 4.2 and 7.1 nm, respectively, based on observeddiffraction peak widths. It should be noted, however, that based on peakshapes, HFC 27 appeared to display a particle size distribution from ˜4nm to 7.1 nm.

The predicted change in the average radius for the Pt₅₁Zn₁₃Fe₃₆ alloy(HFC 32) was a contraction of 3.4% vs. platinum. The determinedcontraction of HFC 32 was about 2.7%. HFC 32 displayed crystallographicordering in the same fashion as noted for HFC 27. The calculatedparticle size for HFC 32 was 4.1 nm. Although this material was slightlymore Fe-rich than HFC 26, the observed particle sizes andelectrochemical performance (see Table D) were similar.

The predicted change in the average radius for the Pt₃₃Zn₂₃Ni₄₄ alloy(HFC 33) was a contraction of 4.6% vs. platinum. The determinedcontraction of HFC 33 was about 2.1%, which may be due an excess ofNi—Zn that was observed. Additionally, HFC 33 appeared to havecrystallographic ordering. The calculated particle size for HFC 33 was3.1 nm.

The predicted change in the average radius for the Pt₅₁Zn₂₀Fe₂₉ alloy(HFC 146) was a contraction of 2.8% vs. platinum. The observed changewas a contraction of about 3.0%. Further, HFC 146 displayed the onset ofcrystallographic ordering and particles of approximately 2.5 nm.

The predicted change in the average radius for the Pt₄₂Zn₂₀Fe₃₈ alloys(HFC 150 and 151) was a contraction of 3.6% vs. platinum. The observedchanges were contractions of about 4.0% and 3.0%, respectively. The XRDpeak shape for the HFC 150 alloy indicated some remaining Pt metal. Boththe HFC 150 and 151 alloys displayed crystallographic ordering andparticles of approximately 3.3 nm.

The predicted change in the average radius for the Pt₄₂Zn₂₀Fe₃₈ alloys(HFC 163, 164, and 165) was a contraction of 3.5% vs. platinum. Theobserved change was a contraction of approximately 3.0%. The XRDanalyses of the HFC 163-165 alloys indicated crystallographic orderingand particle sizes of 3.0, 3.1, and 2.9 nm, respectively.

The predicted change in the average radius for the Pt₃₃Zn₁₂Fe₅₅ alloys(HFC 215 and 216) was a contraction of 5.1% vs. platinum. The observedchange was a contraction of approximately 4.6%. The XRD analyses of HFC215 and 216 indicated crystallographic ordering, again similar to PtFe.However, in this case the lattice parameters are much smaller than PtFewhich is believed to be due to the low amount of platinum of thesematerials. Based on the observed structure and measured stoichiometry,it may be possible that for these particular alloys zinc wassubstituting on the platinum site resulting in an alternativestoichiometry of (Pt_(0.73)Zn_(0.27))Fe. The calculated particle sizesfor HFC 215 and 216 were 6.9 and 3.5 nm, respectively. In the case ofHFC 215, it appears, based on peak shape, that a size distribution ofapproximately 3.5-6.9 nm existed.

In view of the foregoing, for a particular electrocatalyst composition adetermination of the optimum conditions is preferred to produce thehighest activity for that particular composition. For example, thestarting materials used to synthesize the alloy may play a role in theactivity of the synthesized alloy. Specifically, using something otherthan a metal nitrate salt solution to supply the metal atoms may resultin different activities. Different methods of synthesis (e.g., chemicalprecipitation and freeze-drying impregnation) may result in differingactivities due to both differences in particle size and/or differencesin stoichiometric control. Heat treatment parameters such as atmosphere,time, temperature, etc. may also need to be optimized. This optimizationmay involve balancing competing phenomena. For example, increasing theheat treatment temperature is generally known to improve the reductionof a metal salt to a metal which typically increases activity; however,it also tends to increase the size of the electrocatalyst alloy particleand decrease surface area, which decreases electrocatalytic activity.

DEFINITIONS

Activity is defined as the maximum sustainable, or steady state, current(Amps) obtained from the electrocatalyst, when fabricated into anelectrode, at a given electric potential (Volts). Additionally, becauseof differences in the geometric area of electrodes, when comparingdifferent electrocatalysts, activity is often expressed in terms ofcurrent density (A/cm²).

An alloy is a mixture comprising two or more metals. An alloy may bedescribed as a solid solution in which the solute and solvent atoms (theterm solvent is applied to the metal that is in excess) are arranged atrandom, much in the same way as a liquid solution may be described. Ifsome solute atoms replace some of those of the solvent in the structureof the latter, the solid solution may be defined as a substitutionalsolid solution. Alternatively, an interstitial solid solution is formedif a smaller atom occupies the interstices between the larger atoms.Combinations of the two types are also possible. Furthermore, in certainsolid solutions, some level of regular arrangement may occur under theappropriate conditions resulting in a partial ordering that may bedescribed as a superstructure. These solid solutions may havecharacteristics that may be distinguishable through characterizationtechniques such as XRD. Significant changes in XRD may be apparent dueto changes in symmetry, if more complete ordering occurs such as thatwhich occurs between Pt metal and Pt₃Fe. Although the global arrangementof the atoms is extremely similar in both cases, the relationshipbetween the locations of the Pt and Fe atoms is now ordered and notrandom resulting in different diffraction patterns. Further, ahomogeneous alloy is a single compound comprising the constituentmetals. A heterogeneous alloy comprises an intimate mixture of crystalsof individual metals and/or metallic compounds (see, StructuralInorganic Chemistry, A. F. Wells, Oxford University Press, 5^(th)Edition, 1995, chapter 29). An alloy, as defined herein, is also meantto include materials which may comprise elements which are generallyconsidered to be non-metallic. For example, some alloys of the presentinvention may comprise oxygen in atomic, molecular, and/or metallicoxide form. Additionally, some alloys of the present invention maycomprise carbon in atomic, molecular, and/or metal carbide form. Ifpresent, the amount of oxygen and/or carbon in the alloy is typically atwhat is generally considered to be a low level (e.g., less than about 5weight percent), however higher concentrations are also possible (e.g.,up to about 10 weight percent).

It is to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of skill in the art upon reading the above description. The scopeof the invention should therefore be determined not with reference tothe above description alone, but should be determined with reference tothe claims and the full scope of equivalents to which such claims areentitled.

1. An alloy capable of being used as a catalyst in oxidation orreduction reactions, the alloy comprising platinum, zinc and iron,wherein a concentration of platinum is between about 32 and about 50atomic percent, a concentration of zinc that is between about 24 andabout 35 atomic percent, and a concentration of iron is between about 20and about 60 atomic percent.
 2. An alloy capable of being used as acatalyst in oxidation or reduction reactions, the alloy comprisingplatinum, zinc and iron, wherein a concentration of platinum is betweenabout 40 and about 60 atomic percent, a concentration of zinc is between24 and about 30 atomic percent, and a concentration of iron is betweenabout 25 and about 50 atomic percent.
 3. The alloy of claim 1 or 2consisting essentially of platinum, zinc, and iron.
 4. A fuel cellelectrode, the fuel cell electrode comprising electrocatalyst particlesand an electrode substrate upon which the electrocatalyst particles aredeposited, the electrocatalyst particles comprising the alloy of claim 1or
 2. 5. The fuel cell electrode of claim 4 wherein the electrocatalystparticles comprise electrically conductive support particles upon whichthe alloy is dispersed.
 6. The fuel cell electrode of claim 5 whereinthe electrically conductive support particles are selected from thegroup consisting of carbon supports and electrically conductive polymersupports.
 7. A fuel cell comprising an anode, a cathode, a protonexchange membrane between the anode and the cathode, and the alloy ofclaim 1 or 2 as a catalyst for the catalytic oxidation of ahydrogen-containing fuel or the catalytic reduction of oxygen.
 8. Thefuel cell of claim 7 wherein the alloy is dispersed on electricallyconductive support particles.
 9. The fuel cell of claim 7 wherein thealloy is on the surface of the anode and in contact with the protonexchange membrane.
 10. The fuel cell of claim 7 wherein the alloy is onthe surface of the proton exchange membrane and in contact with thecathode.
 11. The fuel cell of claim 7 wherein the alloy is on thesurface of the cathode and in contact with the proton exchange membrane.12. The alloy of claim 1 or 2 wherein the zinc concentration is at least29 atomic %.
 13. The alloy of claim 1 wherein the concentration of theiron is at least 25 atomic %.
 14. The alloy of claim 1 or 2 wherein thealloy is a particulate material.
 15. The alloy of claim 1 or 2 whereinthe alloy is supported on electrically conductive carbon supportparticles.
 16. The alloy of claim 1 or 2 wherein the alloy is supportedon electrically conductive polymer supports.
 17. The alloy of claim 1 or2 wherein the alloy is on the surface of a proton exchange membrane andin contact with a fuel cell anode.
 18. The alloy of claim 1 or 2 whereinthe alloy is an unsupported catalyst layer on a surface of anelectrolyte membrane or on a surface of an electrode.
 19. A method forthe electrochemical conversion of a hydrogen-containing fuel and oxygento reaction products and electricity in a fuel cell comprising an anode,a cathode, a proton exchange membrane therebetween, the alloy of claim 1or 2, and an electrically conductive external circuit connecting theanode and cathode, the method comprising contacting thehydrogen-containing fuel or the oxygen and the alloy to catalyticallyoxidize the hydrogen-containing fuel or catalytically reduce the oxygen.